Quantitative acoustics near the sea floor

For more than a decade the Deep-Tow group of the Marine Physical Laboratory has been developing acoustic systems for near-bottom geophysical studies of the deep sea floor. A major aim of this work has been the detailed measurement of those properties of the sea floor which affect acoustic propagation. Such properties include the slope of the bottom, the acoustic reflectivity of the sea floor and of buried interfaces, and the attenuation of sound in marine sediments. The extreme lateral variability of such properties in the deep ocean makes near-bottom measurements important. This need has resulted in the development of narrow beam altimeters for accurate depth and slope determination, a computerized seismic profiling system for measurement of sea floor reflectivity and attenuation at 4 kHz, and computerized side-scan sonar systems for acoustic backscatter and bottom slope determinations. These systems were designed to provide real-time processing and displays of acquired data. This paper discusses the development and application of these systems, as well as some of the results which illustrate the extreme variability of sea floor acoustic properties which have been observed by these systems.